Method for Consuming Silicon Nanoparticle Film Oxidation

ABSTRACT

A method is provided for consuming oxides in a silicon (Si) nanoparticle film. The method forms a colloidal solution film of Si nanoparticles overlying a substrate. The Si nanoparticle colloidal solution film is annealed at a high temperature in the presence of titanium (Ti). In response to the annealing, Si oxide is consumed in a resultant Si nanoparticle film. In one aspect, the consuming the Si oxide in the Si nanoparticle film includes forming Ti oxide in the Si nanoparticle film. Also in response to a low temperature annealing, solvents are evaporated in the colloidal solution film of Si nanoparticles. Si and Ti oxide molecules are sintered in the Si nanoparticle film in response to the high temperature annealing.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention generally relates to integrated circuit (IC) fabricationand, more particularly, to a method of consuming oxidation that occursin the formation of a silicon (Si) nanoparticle film.

2. Description of the Related Art

Low temperature and low thermal budget processing for large areaelectronics is gaining momentum with many recent advances involvingnon-vacuum deposition, etching steps (such as printing), androll-to-roll, low-cost techniques on flexible substrates. One way toform the active semiconductor film for a thin-film transistor in such aprinting-compatible process is by the use of Si, or other semiconductor,in the form of a nanoparticle liquid suspension (termed nanofluidsuspension). Particle sizes for Si can range from <1 nm to >100 nm,based on the desired application. This process typically involves theselective deposition of the nanofluid, and one or more elevatedtemperature steps aiming at the evaporation of the various solvents ofthe suspension and the sintering of nanoparticles to form aninterconnected porous nanoparticle film and/or a homogeneoussemi-conducting film.

One problem common with the processing of Si nanofluids is thespontaneous formation of silicon dioxide on the surface of thenanoparticles. A thin oxide forms within seconds after exposure in airin ambient temperature. Thicker oxides form during the aforementionedelevated temperature steps, if they take place in an oxidizingenvironment. This oxide can easily reach thicknesses that severely limitthe conductivity of the semiconductor, and can also severely limit thesintering process. One solution to this problem is to prevent theexposure of the nanofluid to an oxidizing environment until sintering isaccomplished. While feasible, this certainly entails increasing thelevel of complexity and sophistication of the processing steps (i.e.processing in low pressure, inert gas atmosphere, etc.), which runscounter to the quick-and-inexpensive target of this approach.

It would be advantageous if semiconductor nanoparticles films could beformed in a low complexity process, without forming semiconductor oxideproducts.

SUMMARY OF THE INVENTION

Disclosed herein is a process for the fabrication of a non-self-alignedtop-gate MOS field effect transistor using a sintered Si nanoparticlefilm for the active layer. The nanoparticle film is coated on thesubstrate with spin coating or another suitable method, such asextrusion coating, spraying, printing, etc. The nanoparticle film iscoated in a colloidal suspension form. Typically, one low-temperatureannealing step (soft-bake) is used to evaporate the solvent of thedeposited film, followed by a higher temperature anneal step, used tosinter the film.

A unique part of the process is the deposition of a thin titanium (Ti)layer on top of the Si nanoparticle film prior to the soft-bake step, orprior to the final annealing (sintering) step. In another aspect of theinvention, the colloidal solution contains, along with the Sinanoparticles, a controlled amount of Ti nanoparticles.

Accordingly, a method is provided for consuming oxides in a silicon (Si)nanoparticle film. The method forms a colloidal solution film of Sinanoparticles overlying a substrate. The Si nanoparticle colloidalsolution film is annealed in the presence of titanium (Ti). In responseto the annealing, Si oxide is consumed in a resultant Si nanoparticlefilm. In one aspect, the consuming the Si oxide in the Si nanoparticlefilm includes forming Ti oxide in the Si nanoparticle film. Also inresponse to the annealing, Si and Ti oxide molecules are sintered in theSi nanoparticle film.

Additional details of the above-described method, an associatedthin-film transistor (TFT) fabrication method, and a TFT with a Sinanoparticle active layer film including titanium (Ti) oxide, areprovided below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial cross-sectional view of a thin-film transistor (TFT)with a silicon (Si) nanoparticle active layer film including titanium(Ti) oxide.

FIGS. 2A through 2C show a schematic sequence for the nanoparticle filmdeposition process.

FIG. 3 depicts a sequence of steps being performed in the fabrication ofa TFT made using a semiconductor nanoparticle film process that consumesoxides.

FIG. 4 is a graph of the I_(D)-V_(G) characteristic for four NMOS Sinanoparticle TFTs with Ti incorporated into the nanoparticle film.

FIG. 5 shows a family of I_(D)-V_(D) curves from a representativedevice.

FIG. 6 is a flowchart illustrating a method for consuming oxides in asilicon Si nanoparticle film.

FIG. 7 is a flowchart illustrating a method for consuming oxidation in aSi nanoparticle active layer film, in the fabrication of a TFT.

DETAILED DESCRIPTION

FIG. 1 is a partial cross-sectional view of a thin-film transistor (TFT)with a silicon (Si) nanoparticle active layer film including titanium(Ti) oxide. The TFT 100 comprises a substrate 102, which may be atransparent material such as glass or a semiconductor. A Si nanoparticleactive layer film 104, including Ti oxide, overlies the substrate 102.In some aspects, not shown, an insulator or basecoat may be formedinterposed between the Si nanoparticle active layer film 104 and thesubstrate 102. A gate insulator 106 overlies the Si nanoparticle activelayer film 104. A source region 108 and a drain region 110 are formed inthe Si nanoparticle active layer film 104. A gate electrode 112 overliesthe gate insulator 106. The Si nanoparticle active layer film 104includes no Si oxide. In one aspect, the Si nanoparticle active layerfilm 104 may include Ti silicide.

Functional Description

The process used to form the TFT of FIG. 1 incorporates a small amountof Ti with the deposited Si nanoparticle film prior to the sinteringstep. The amount of Ti is determined by the amount of Si nanoparticlespresent in the film. The purpose is to have the Ti react with the all ormost of the residual SiO₂ present in the film. An excessive amount of Tireacts with, and consumes entire Si nanoparticles, forming titaniumsilicide, which is not necessarily desirable. These reactions take placeduring the main annealing (sintering) step. In one aspect of the Sinanoparticle film treatment, Ti is present in the form of nanoparticlesin the Si particle solution. In another aspect, a thin Ti film isdeposited on top of the silicon nanoparticle film, after the Sinanoparticle film is deposited on the substrate and a preliminarysolvent evaporation anneal step is performed, prior to the final, highertemperature sintering step.

FIGS. 2A through 2C show a schematic sequence for the nanoparticle filmdeposition process. FIG. 2A shows nanofluid coating prior to solventevaporation. FIG. 2B shows a the Ti film top coating. FIG. 2C depictsthe formation of the Si nanoparticle active semiconductor film aftersintering, in which state the Ti has reacted with the insulating SiO₂areas forming electrically conductive Ti oxide and Ti silicide.

FIG. 3 depicts a sequence of steps being performed in the fabrication ofa TFT made using a semiconductor nanoparticle film process that consumesoxides. In Step 1 the process begins with a suitable substrate (glass,metal or plastic foil, etc.), with a basecoat already deposited, if soneeded. The combined substrate-basecoat system is referred to as thesubstrate 102. Then, in Step 2 a semiconductor nanoparticle film 104 isdeposited, typically from a colloidal solution source, and an initialsolvent-evaporation annealing step is performed. The nanoparticleformulation may already contain an amount of titanium nanoparticles(Step 2B). In another realization, as shown in FIG. 2A, a thin titaniumfilm 300 is deposited on top of the semiconductor nanoparticle film 104.

Next, in Step 3 the nanoparticle semiconductor film is sintered, eitherwith a low thermal budget process ETA, or laser), or with a traditionalfurnace anneal. The reaction between Ti and the residual semiconductoroxide in the nanoparticle film occurs in this step. In Step 4 the filmis then cleaned, and residual (if any) Ti is etched away. In Step 5 athin silicon dioxide or silicon nitride film 302 is then deposited andpatterned, forming a gate insulator to protect the TFT channel fromsubsequent etch steps. This is followed by the TFT island patterningshown in Step 6. The source 108 and drain 110 regions are then depositedand patterned in Step 7. They are typically in-situ doped n+(for NMOS)or p+ (for PMOS) amorphous Si. Then, the gate oxide 106 is deposited inStep 8, followed by contact hole opening and the deposition andpatterning of the gate, and the deposition of source and drain metal 304in Step 9.

Experimental Results

A two-wafer lot experiment was run to test the above-describedfabrication sequence. Both wafers used a nanoparticle Si ink formulatedat Nanogram®. The first wafer was run with the sequence described aboveand shown in FIG. 3. The second wafer was run with the same sequence,without including Step 2 as shown in FIG. 3, i.e. without depositing aTi film on top of the nanoparticle active film, prior to the sinteringin Step 3.

The Si nanoparticle film was spin-coated on the substrate, andsoft-baked on a hotplate at 150° C. for 10 minutes to evaporate the inksolvent. For the first wafer, 50 Å of Ti were deposited via magnetronsputtering on top of the film. The second (control) wafer did not gothrough this step. Then, both wafers were sintered with an RTA anneal at900° C. for 5 minutes. The etch-stop channel-protecting oxide was thendeposited, 50 nanometers (nm) thick, and patterned. After patterning theactive semiconductor film, the in-situ doped n+Si for the source/drainterminals was deposited via plasma-enhanced chemical vapor deposition(PECVD), 100 nm thick. The dopants of this film were furnace activatedwith a furnace anneal at 650° C. for five hours. After patterning the n+the source and drain, a 200 nm thick SiO₂ film was deposited via PECVD,and contacts for the drain and source were dry etched in plasma.Finally, the top metal was deposited via sputtering on top, comprisingof a Ti—Al stack (50 nm Al, 300 nm Al), and patterned. The devices werethen ready for measurement.

The oxide consuming process films (i.e. incorporating Ti into thenanoparticle film) produced working TFTs, albeit with very low ONcurrents. However, the wafer fabricated without the Ti deposition stepdid not produce any working devices at all drain current was negligibleand could not be controlled by the gate voltage. This shows that theoxide consuming process can improve to a degree the performance of a Sinanoparticle film. Of course, more practical devices need furtheroptimized.

FIG. 4 is a graph of the I_(D)−V_(G) characteristic for four NMOS Sinanoparticle TFTs with Ti incorporated into the nanoparticle film.W/L=80 μm/40 μm; V=20 V.

FIG. 5 shows a family of I_(D)-V_(D) curves from a representativedevice. One obvious issue limiting mobility of the fabricated devices isthe high drain and source contact resistance, as it is evident frominspection of the linear region operation curves.

Although device performance is low, there is evidence of transistoraction. Identically processed TFTs without the incorporated metal intothe active nanoparticle film did not have transistor functionality. Anoptimized process should help avoid the limitations of usingnanoparticle suspensions for TIT fabrication in air atmosphere.

FIG. 6 is a flowchart illustrating a method for consuming oxides in a Sinanoparticle film. Although the method is depicted as a sequence ofnumbered steps for clarity, the numbering does not necessarily dictatethe order of the steps. It should be understood that some of these stepsmay be skipped, performed in parallel, or performed without therequirement of maintaining a strict order of sequence. Generallyhowever, the method follows the numeric order of the depicted steps. Themethod starts at Step 600.

Step 602 provides a substrate. Step 604 forms a colloidal solution filmof Si nanoparticles overlying the substrate. In one aspect, Step 604forms the colloidal solution film of Si nanoparticles in an ambient airenvironment including oxygen. This step forms silicon oxide in the Sinanoparticles. Step 604 uses a deposition method such as spin coating,extrusion coating, spraying, or printing. The Si nanoparticles typicallyhave a diameter in the range from 10 to 100 nanometers (nm).

In one aspect in response to a soft-bake or low temperature annealing(Step 605 b), Step 605 c evaporates solvents in the colloidal solutionfilm of Si nanoparticles.

Step 606 anneals the Si nanoparticle colloidal solution film in thepresence of titanium (Ti). Step 606 may use a laser annealing, furnaceannealing, or rapid thermal annealing (RTA) method. In response to theannealing, Step 608 consumes Si oxide in a resultant Si nanoparticlefilm. In one aspect, Step 608 also forms Ti oxide in the Si nanoparticlefilm. In another aspect, Step 608 forms Ti silicide in the Sinanoparticle film.

Step 608 further sinters Si and Ti oxide molecules in the Sinanoparticle film. Sintering is generally understood to be a method tomake a solid film from powders based upon the principle of atomicdiffusion. In this most sintering process, the Si nanoparticle powderedmaterial is heated to a temperature below the melting point. The atomsin the powder particles diffuse across the boundaries of the particles,fusing the particles together and creating one solid piece.

In one variation, forming the colloidal solution film of Sinanoparticles overlying the substrate in Step 604 includes adding Tinanoparticles to the colloidal solution of Si nanoparticles prior todeposition over the substrate. Alternatively, prior to annealing in Step606, Step 605 a deposits a layer of Ti film overlying the colloidalsolution film of Si nanoparticles. Note: Step 605 c may be performedbefore or after Steps 605 b and 605 c.

FIG. 7 is a flowchart illustrating a method for consuming oxidation in aSi nanoparticle active layer film, in the fabrication of a TFT. Themethod begins with Step 700. Step 702 provides a substrate. Step 704forms a colloidal solution film of Si nanoparticles overlying thesubstrate. In one aspect, Step 704 forms the colloidal solution film ofSi nanoparticles in an ambient air environment including oxygen. Thecolloidal solution film of Si nanoparticles can be deposited using aspin coating, extrusion coating, spraying, or printing method.Typically, the Si nanoparticles have a diameter in the range from 10 to100 nm.

Step 706 anneals the Si nanoparticle colloidal solution film in thepresence of Ti, forming a Si nanoparticle active layer film. Step 706may use a laser annealing, furnace annealing, or RTA method. In responseto the annealing, Step 708 consumes Si oxide in the Si nanoparticleactive layer film. Step 710 forms a gate insulator overlying the Sinanoparticle active layer film. Step 712 forms source and drain regionsin the Si nanoparticle active layer film. Step 714 forms a gateelectrode overlying the gate insulator.

In one aspect, consuming the Si oxide in the Si nanoparticle activelayer film (Step 708) includes forming Ti oxide in the Si nanoparticleactive layer film. In another aspect, Step 708 sinters Si and Ti oxidemolecules in the Si nanoparticle active layer film.

In one aspect, forming the colloidal solution film of Si nanoparticlesoverlying the substrate in Step 704 includes adding Ti nanoparticles tothe colloidal solution of Si nanoparticles prior to deposition over thesubstrate. Alternatively, prior to annealing in Step 706, Step 705deposits a layer of Ti film overlying the colloidal solution film of Sinanoparticles.

A method for consuming oxide in a Si nanoparticle film has beenprovided, as well as a TFT made using the method. Examples of particularprocess steps have been presented to illustrate the invention. However,the invention is not limited to merely these examples. While Si is theonly semiconductor material mentioned, the process also consumes theoxides in other, unnamed semiconductor materials. Other variations andembodiments of the invention will occur to those skilled in the art.

We claim:
 1. A method for consuming oxides in a silicon (Si)nanoparticle film, the method comprising: providing a substrate; forminga colloidal solution film of Si nanoparticles overlying the substrate;annealing the Si nanoparticle colloidal solution film in the presence oftitanium (Ti); and, in response to the annealing, consuming Si oxide ina resultant Si nanoparticle film.
 2. The method of claim 1 whereinconsuming the Si oxide in the Si nanoparticle film includes forming Tioxide in the Si nanoparticle film.
 3. The method of claim 2 whereinconsuming Si oxide in the Si nanoparticle film includes sintering Si andTi oxide molecules in the Si nanoparticle film.
 4. The method of claim 2wherein consuming Si oxide in the Si nanoparticle film includes formingTi silicide in the Si nanoparticle film.
 5. The method of claim 1wherein forming the colloidal solution film of Si nanoparticlesoverlying the substrate includes forming the colloidal solution film ofSi nanoparticles in an ambient air environment including oxygen.
 6. Themethod of claim 1 wherein forming the colloidal solution film of Sinanoparticles overlying the substrate includes adding Ti nanoparticlesto the colloidal solution of Si nanoparticles prior to deposition overthe substrate.
 7. The method of claim 1 further comprising: prior toannealing, depositing a layer of Ti film overlying the colloidalsolution film of Si nanoparticles.
 8. The method of claim 1 whereinforming the colloidal solution film of Si nanoparticles overlying thesubstrate includes depositing the colloidal solution film of Sinanoparticles using a method selected from a group consisting of spincoating, extrusion coating, spraying, and printing.
 9. The method ofclaim 1 wherein forming the colloidal solution film of Si nanoparticlesoverlying the substrate includes forming a colloidal solution film withSi nanoparticles having a diameter in a range from 10 to 100 nanometers(nm).
 10. The method of claim 1 wherein annealing includes using methodselected from a group consisting of laser annealing, furnace, and rapidthermal annealing (RTA).
 11. In the fabrication of a thin-filmtransistor (TFT), a method for consuming oxidation in a silicon (Si)nanoparticle active layer film, the method comprising: providing asubstrate; forming a colloidal solution film of Si nanoparticlesoverlying the substrate; annealing the Si nanoparticle colloidalsolution film in the presence of titanium (Ti), forming a Sinanoparticle active layer film; in response to the annealing, consumingSi oxide in the Si nanoparticle active layer film; forming a gateinsulator overlying the Si nanoparticle active layer film; formingsource and drain regions in the Si nanoparticle active layer film; and,forming a gate electrode overlying the gate insulator.
 12. The method ofclaim 11 wherein consuming the Si oxide in the Si nanoparticle activelayer film includes forming Ti oxide in the Si nanoparticle active layerfilm.
 13. The method of claim 11 wherein consuming silicon oxide in theSi nanoparticle active layer film includes sintering Si and Ti oxidemolecules in the Si nanoparticle active layer film.
 14. The method ofclaim 11 wherein forming the colloidal solution film of Si nanoparticlesoverlying the substrate includes forming the colloidal solution film ofSi nanoparticles in an ambient air environment including oxygen.
 15. Themethod of claim 11 wherein forming the colloidal solution film of Sinanoparticles overlying the substrate includes adding Ti nanoparticlesto the colloidal solution of Si nanoparticles prior to deposition overthe substrate.
 16. The method of claim 11 further comprising: prior toannealing, depositing a layer of Ti film overlying the colloidalsolution film of Si nanoparticles.
 17. The method of claim 11 whereinforming the colloidal solution film of Si nanoparticles overlying thesubstrate includes depositing the colloidal solution film of Sinanoparticles using a method selected from a group consisting of spincoating, extrusion coating, spraying, and printing.
 18. The method ofclaim 11 wherein forming the colloidal solution film of Si nanoparticlesoverlying the substrate includes forming a colloidal solution film withSi nanoparticles having a diameter in a range from 10 to 100 nanometers(nm).
 19. The method of claim 11 wherein annealing includes using amethod selected from a group consisting of laser annealing, furnace, andrapid thermal annealing (RTA).
 20. A thin-film transistor (TFT) with asilicon (Si) nanoparticle active layer film including titanium (Ti)oxide, the TFT comprising: a substrate; a Si nanoparticle active layerfilm, including Ti oxide, overlying the substrate; a gate insulatoroverlying the Si nanoparticle active layer film; source and drainregions in the Si nanoparticle, active layer film; and, a gate electrodeoverlying the gate insulator.
 21. The TFT of claim 20 wherein the Sinanoparticle active layer film includes no Si oxide.
 22. The TFT ofclaim 20 wherein the Si nanoparticle film includes Ti silicide.